Hank, I normally don't have a test performed. Depending on your geographic area it can give a clue to what type of soil you have and the need. Most (if not all) building inspectors in NC & VA "probe" the soil and based upon the depth of the penetration make a determination. When they question the probing they require a soil compaction test. My practical experience in areas of poor soil the remedy is two or three # 4 bars of re-bar in footing. I'm just completing a 2000 sq ft on a 1300 sq ft basement that I did re bar just a extra measure. No need just above the min. code requirements. I normally pay around $300 from the engineering firm that I used periodically. I'm sure some will disagree on the methodology and science (or lack of it).

Re-bar is cheap insurance and not a bad thing to do anyway. Years ago I dug many foundation holes and ran a compactor around and that was all that was needed as long as you were on virgin soil. If you have to bring in fill to level a foundation hole, that is where you need testing.

These guys give good advice Hank, on virgin ground, soil compact tests are kind of reinventing the wheel, as local codes account for local soil compression. The answer to any questionable scenario is re-bar, so just add it if you have doubt.

Frost is irrelavant, even in slabs, as code has you go below 24" or more for footers ( under frost level).

The only problems with foundation I am familiar with in my experience,(not my project thank God) was a house built to code, but built on a coal mine filled in 30 years prior.

Ray you are correct on your answer. I know in NC in my zone we only have to go 8 inches below frost. I have heard of some zones that require 36 inches. In Southern VA near the Blue Ridge its 18 inches and two bars of # 4 are required. If you take a probe and you can penetrate 2-4 inches into virgin soil I would be installing rebar. If It is significantly more than that a soil scientist should be consulted.